One particularly advantageous application for a power supply of this type is its utilization in penetration-type polychrome cathode-ray tubes used for equipping visual-display consoles. In such penetration tubes, the screen is constituted by multiple layers which, in accordance with current practice, consist of two or three fluorescent layers of different colors separated by one or two barrier layers. Depending on the final acceleration voltage applied to the electron beam, the electrons excite one of the color layers and induce the appearance of the corresponding color.
Thus, only the fluorescent layer located on the side nearest the electron gun is excited and a red color appears in the case of a low acceleration voltage of the order of 10 kV. In the case of a higher acceleration voltage such as 18 kV, the fluorescent layer on the front side of the screen is excited, thus displaying a green color. In the case of intermediate voltage values, a mixture of the two colors just mentioned is obtained in a two-layer tube. In a tube having three fluorescent layers, the number of colors and of color mixtures is greater.
It is therefore apparent that, in order to present colored images with penetration tubes, it is necessary to pass rapidly from one value of the very-high-voltage supply to another. In fact, a polychrome penetration tube cannot emit a number of different colors at the same time and is capable of doing so only sequentially at the rate of switching of the anode voltage. Retinal persistence, however, enables the eye to see a single image in several colors.
The change of color in the case of a polychrome tube is produced by variation of its anode voltage. In the case of electromagnetic deflection, the angle of deflection of the electron beam then varies in inverse ratio to the square root of the anode voltage. Scanning is corrected as a function of this law so as to ensure that the variation in anode voltage which is necessary for operation of the tube does not cause any variations in the dimensions of the image.
The energy of an electron differs according to the anode voltage and therefore according to the color. Furthermore, the efficiency of the phosphor which forms the layers is different; it appears necessary to correct the anode current in order to obtain a luminance which does not vary as a function of the colors formed on the screen of the tube.
Correction of the electron-beam deflection and of the luminance of the cathode-ray tube does not involve any difficulties for which no acceptable solutions have been found.
On the other hand, the variations in anode voltage do present problems arising from the fact that the voltage to be switched is of high value (of the order of at least 8 kV), and that this switching operation must take place in a short period of time in order to leave a sufficient image trace interval and is performed with a relatively high anode capacitance of the order of at least 500 pf.
The value of the anode voltage must therefore be very accurate in order to ensure correct positioning of the spot on the screen.
Different solutions have already been proposed for the design of a switchable very-high-voltage power supply.
One system makes use of a vacuum tube adapted to carry out a series regulation for charging the capacitor of the polychrome tube while another such tube serves to discharge the capacitor at the moment of switching operations. This device has a disadvantage in that it does not permit short switching times.
Another system, which can be considered an improvement over the foregoing one, utilizes two pulse transformers for reducing the rise time or decay time of transients. A first transformer delivers a positive pulse of the order of 8 kV for charging the tube capacitor to a level of the order of 10 to 18 kV. After the steady-level state has been reached, a second transformer delivers a negative pulse of the order of 8 kV in order to reduce the charge on the tube capacitor from 18 kV to 10 kV. One of the disadvantages of this arrangement lies in the fact that, in order to obtain intermediate anode voltages which make it possible to have colors other than the basic primary colors, the number of these circuit components must be increased according to the number of voltages to be switched. Under these conditions, the system rapidly becomes cumbersome and costly.
A further system provides a number of high-voltage generators corresponding to the number of cathode-ray-tube operating voltages to be established. These voltages are selected by a switch. In a system of this type, however, the switch must be capable of holding the voltage to be switched and of operating at sufficiently high speed. An electromechanical device can be employed but it is preferable to adopt an electronic device constituted by a certain number of transistors or of thyristors. However, a switch of this type which maintains the selected voltage in each position thereof is not easy to construct.